Polishing Composition and Polishing Method

ABSTRACT

A first polishing composition includes abrasive grains and an iodine compound and has a pH of 6 or more. The first polishing composition can suitably polish the Si [0001] plane of a single crystal silicon carbide substrate. A second polishing composition includes an iodine compound and has a pH of 8 or less. The second polishing composition can suitably polish the C [000-1] plane of a single crystal silicon carbide substrate. A third polishing composition includes abrasive grains and an iodine compound and has a pH of 6 to 8, inclusive. The third polishing composition can suitably polish each of the Si [0001] and C [000-1] planes of a single crystal silicon carbide substrate.

BACKGROUND OF THE INVENTION

The present invention relates to a polishing composition for use in polishing an object formed of single crystal semiconductor, in particular an object formed of single crystal silicon carbide, such as a substrate of single crystal hexagonal silicon carbide, and the polishing method using the polishing composition.

A substrate of single crystal silicon carbide, such as a single crystal 4H-SiC or 6H-SiC substrate is normally polished by a preliminary step for polishing the substrate surface with a slurry containing abrasive grains of diamond, and finishing step for polishing the preliminarily polished surface. The finishing step removes an amorphous polishing-modified layer developing on the substrate surface in the preliminary step and, at the same time, flattens the exposed surface into a crystal plane as a result of removal of the polishing-modified layer. A polishing composition containing colloidal silica and used at a pH of 7 to 10, disclosed by Japanese Laid-Open Patent Publication No. 2004-299018, is one of the known polishing compositions for the finishing polishing step. This composition, however, involves problems resulting from needing a very long polishing time, because of its poor capability of polishing crystal planes.

A substrate of single crystal 4H-SiC or 6H-SiC as single crystal hexagonal silicon carbide has two planes of different orientation, Si [0001] and C [000-1] planes. Oxidation and etching proceed at a lower rate on the Si [0001] plane of single crystal 4H-SiC or 6H-SiC substrate than on other planes, e.g., C [000-1] plane. Therefore, it is difficult for a conventional method to polish the single crystal Si [0001] plane at a high removal rate. Moreover, the polishing composition disclosed by the above patent publication also involves problems of easily causing surface defects, e.g., pit, when applied to the C [000-1] plane of single crystal 4H-SiC or 6H-SiC substrate.

Still more, a single crystal hexagonal silicon carbide substrate must be polished separately for the Si [0001] and C [000-1] planes, which have very different chemical and mechanical properties, with two different polishing compositions, each suitable for each plane. However, there are demands for polishing compositions which can singularly polish these planes to improve work efficiency.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a polishing composition which can suitably polish the Si [0001] plane, and a polishing method using the composition. A second object of the present invention is to provide a polishing composition which can suitably polish the C [000-1] plane, and a polishing method using the composition. A third object of the present invention is to provide a polishing composition which can suitably polish each of the Si [0001] and C [000-1] planes singularly, and a polishing method using the composition.

To achieve the foregoing objectives and in accordance with a first aspect of the present invention, a polishing composition including abrasive grains and an iodine compound and having a pH of 6 or more is provided.

In accordance with a second aspect of the present invention, a polishing composition including an iodine compound and having a pH of 8 or less is provided.

In accordance with a third aspect of the present invention, a polishing composition including abrasive grains and an iodine compound and having a pH of 6 to 8, inclusive is provided.

In accordance with a fifth aspect of the present invention, a method for polishing an object formed of single crystal silicon carbide is provided. The method includes:

preparing any one of the polishing composition; and polishing the object using the prepared polishing composition.

Other aspects and advantages of the invention will become apparent from the following description, illustrating by way of example the principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first and second embodiments of the present invention will be described below.

The polishing composition of the first embodiment is produced by incorporating colloidal silica with a given quantity of iodate or periodate, and, as required, diluting the resulting mixture with water. It is therefore essentially composed of colloidal silica working as abrasive grains, iodate or periodate as an iodine compound and water.

The polishing composition of the second embodiment is produced by incorporating colloidal silica with a given quantity of iodic or periodic acid and an alkali, and, as required, diluting the resulting mixture with water. It is therefore essentially composed of colloidal silica working as abrasive grains, an iodic or periodic acid as an iodine compound, alkali as a pH adjuster and water. In other words, the polishing composition of the second embodiment differs from that of the first embodiment in that it contains an iodic or periodic acid as an iodine compound in place of iodate or periodate, and an alkali.

The polishing compositions of the first and second embodiments are used for polishing a single crystal 4H-SiC or 6H-SiC substrate, more specifically the Si [0001] plane of a single crystal 4H-SiC or 6H-SiC substrate.

Each of the polishing compositions of the first and second embodiments may fail to exhibit a sufficient polishing capability when it contains colloidal silica at below 0.05% by mass, or more specifically below 0.1% by mass, or even more specifically below 1% by mass. Therefore, it preferably contains colloidal silica at 0.05% by mass or more, more preferably 0.1% by mass or more, most preferably 1% by mass or more, in order to achieve a higher removal rate. On the other hand, it is uneconomical when it contains colloidal silica at above 50% by mass, or more specifically 48% by mass, or even more specifically 45% by mass, because colloidal silica with a higher content than commercial colloidal silica is needed for production of the polishing composition. Therefore, each of the polishing compositions of the first and second embodiments contains colloidal silica preferably at 50% by mass or less, more preferably 48% by mass or less, most preferably 45% by mass or less.

Colloidal silica having an average primary particle size below 5 nm, or more specifically 15 nm, or even more specifically more below 25 nm does not have a sufficient polishing capability for the Si [0001] plane. Therefore, colloidal silica for each of the polishing compositions of the first and second embodiments preferably has an average primary particle size of 5 nm or more, more preferably 15 nm or more, most preferably 25 nm or more, for the composition to achieve a higher removal rate. On the other hand, colloidal silica having an average primary particle size above 120 nm, or more specifically above 100 nm, or even more specifically above 85 nm has to be incorporated at a fairly high content in each of the polishing compositions of the first and second embodiments, for the composition to exhibit a sufficient polishing capability. Therefore, it preferably has an average primary particle size of 120 nm or less, more preferably 100 nm or less, most preferably 85 nm or less, in order to reduce cost of the polishing composition. The average primary particle size of colloidal silica may be determined from the relative surface area measured by, e.g., the BET method.

The iodine compound incorporated in the polishing composition of the first embodiment to improve its removal rate may be an iodate or periodate, preferably the latter, more preferably sodium metaperiodate (NaIO₄). A periodate is preferable to an iodate, because of its higher redox potential and higher oxidizing power. Sodium metaperiodate is preferable to any other periodate, because of its wider availability.

The iodine compound incorporated in the polishing composition of the second embodiment to improve its removal rate may be an iodic or periodic acid, preferably the latter, more preferably orthoperiodic acid (H₅IO₆). A periodic acid is preferable to an iodic acid, because of its higher redox potential and higher oxidizing power.

Orthoperiodic acid is preferable to any other periodic acid, because of its wider availability.

Each of the polishing compositions of the first and second embodiments may fail to polish the Si [0001] plane at sufficiently high removal rate when it contains an iodine compound at below 0.1 g/L, or more specifically below 0.5 g/L, or even more specifically below 1 g/L. Therefore, it preferably contains an iodine compound at 0.1 g/L or more, more preferably 0.5 g/L or more, most preferably 1 g/L or more, in order to achieve a higher removal rate. On the other hand, when each of the polishing compositions of the first and second embodiments contains an iodine compound at above 500 g/L, or more specifically above 250 g/L, or even more specifically above 100 g/L, it may deteriorate a polishing pad faster. Moreover, a precipitate may be formed in the polishing composition when the iodine compound is an iodate or periodate. In order to avoid these problems, each of the polishing compositions of the first and second embodiments contains an iodine compound preferably at 500 g/L or less, more preferably 250 g/L or less, most preferably 100 g/L or less.

An alkali to be incorporated in the polishing composition of the second embodiment is preferably a lithium compound, e.g., lithium hydroxide (LiOH) or an inorganic lithium salt, or ammonia (NH₃), more preferably lithium hydroxide or ammonia to make the composition have a good storage stability. Specifically, the inorganic lithium salt includes chloride, lithium carbonate, lithium nitrate, lithium sulfate and lithium phosphate, of which lithium chloride and lithium carbonate are more available.

When the polishing composition of the second embodiment contains an alkali at below 0.1 g/L, or more specifically below 0.5 g/L, or even more specifically below 1 g/L, it may have deteriorated polishing capability for the Si [0001] plane, because of insufficient pH level of the composition. Therefore, it preferably contains an alkali at 0.1 g/L or more, more preferably 0.5 g/L or more, most preferably 1 g/L or more, in order to achieve a higher removal rate. On the other hand, when it contains an alkali at above 20 g/L, or more specifically above 15 g/L, or even more specifically above 10 g/L, it may have deteriorated storage stability. Therefore, it contains an alkali preferably at 20 g/L or less, more preferably 15 g/L or less, most preferably 10 g/L or less, in order to have good storage stability.

It is essential that each of the polishing compositions of the first and second embodiments is kept at a pH of 6 or more to suitably polish the Si [0001] plane. It should be noted, however, that each of the polishing compositions tends to have a polishing capability decreasing as pH level decreases, even when it is 6 or more. Therefore, each composition is preferably kept at a pH of 7 or more, in order to achieve a higher removal rate. On the other hand, when each of the polishing compositions of the first and second embodiments is kept at an excessively high pH level, it may have colloidal silica dissolved in the composition. Therefore, it is preferably kept at a pH of 14 or less, more preferably 13 or less, most preferably 12 or less viewed from prevention of colloidal silica dissolution.

The first and second embodiments bring the following advantages.

Each of the polishing compositions of the first and second embodiments can polish the Si [0001] plane of a single crystal 4H-SiC or 6H-SiC substrate faster than a conventional composition, conceivably by virtue of the iodine compound, which is iodic acid, periodic acid or a salt thereof, exhibiting a sufficient oxidizing power in a pH region of 6 or more to oxidize the plane.

The polishing composition of the second embodiment contains an alkali. It is therefore not difficult to keep the composition at a pH of 6 or more, even when it contains an acid as an iodine compound, such as iodic or periodic acid.

Each of the polishing compositions of the first and second embodiments may be modified in the following manner.

Colloidal silica as the abrasive grains for these compositions may be replaced by another species of silica, e.g., fumed silica, or alumina or chromium oxide. However, silica (in particular colloidal silica) is preferably used, because it tends to leave less damage on the polished surface than alumina or chromium oxide.

The polishing composition of the first embodiment may be incorporated with an alkali as a pH adjuster, as required. The preferable species and content of the alkali for the first embodiment are the same as those for the second embodiment.

Incorporation of an alkali is not essential for the polishing composition of the second embodiment. In other words, the polishing composition of the second embodiment may be essentially composed of colloidal silica, an iodic or periodic acid and water. However, an alkali is preferably incorporated to achieve a higher removal rate. It is essential to keep the polishing composition at a pH of 6 or more, even when it is alkali-free.

Each of the polishing compositions of the first and second embodiments may be incorporated with one or more known additives, e.g., corrosion inhibitor or defoaming agent.

Each of the polishing compositions of the first and second embodiments may be used for polishing a plane other than the Si [0001] plane of a single crystal 4H-SiC or 6H-SiC substrate. Moreover, it may be used for polishing an object formed of single crystal cubic silicon carbide, e.g., a 3C-SiC substrate. It may be used to polish an object formed of single crystal silicon carbide other than a single crystal silicon carbide substrate, or an object other than the object formed of single crystal silicon carbide. However, it is stressed that each of the polishing compositions of the first and second embodiments has a higher polishing capability for an object formed of single crystal silicon carbide, in particular the Si [0001] plane of a single crystal 4H-SiC or 6H-SiC substrate than a conventional composition. It is therefore preferably used for polishing an object formed of single crystal silicon carbide, more preferably the Si [0001] plane of a single crystal 4H-SiC or 6H-SiC substrate.

Next, Examples and Comparative examples related to the polishing compositions of the first and second embodiments will be described.

Colloidal silica sol, an iodine compound or alternative compound and a pH adjuster were adequately mixed, and the mixture was diluted with water as required to prepare a polishing composition in each of Examples 1 to 11 and Comparative examples 1 to 13. These components are given in detail in Table 1 together with composition pH levels. The colloidal silica for these compositions had an average primary particle size of 35 nm.

The Si [0001] plane of a single crystal silicon carbide substrate was polished with each of the polishing compositions prepared in Examples 1 to 11 and Comparative examples 1 to 13 under the polishing conditions given in Table 2. The substrate was weighed before and after the polishing to determine removal rate, given in the “removal rate” column in Table 1.

A given quantity (100 mL) of each of the polishing compositions prepared in Examples 1 to 11 and Comparative examples 1 to 13, put in a 250 mL polyethylene container, was allowed to stand at room temperature for 3 months to observe changed external appearances periodically, to evaluate their storage stability in accordance with the following standards:

Excellent: No changes such as gelation, precipitation and viscosity increase were observed in 3 months, marked with E, and Good: No changes were observed in 1 month, but such changes were observed in 3 months, marked with G. The results are given in the “stability” column in Table 1.

TABLE 1 Iodine compound Colloidal or alternative silica compound pH adjuster Content Content Content Removal rate [mass %] Name [g/L] Name [g/L] pH [nm/hr.] Stability Ex. 1 40 NaIO₄ 5 — 0 8.2 36.2 G Ex. 2 40 NaIO₄ 10 — 0 7.8 45.4 G Ex. 3 40 NaIO₄ 15 — 0 7.5 54.5 G Ex. 4 40 NaIO₄ 10 LiOH 1.8 10.8 59.4 G Ex. 5 40 H₅IO₆ 10 LiOH 1.8 9.0 48.0 G Ex. 6 40 H₅IO₆ 10 LiOH 0.8 6.5 30.2 G Ex. 7 40 H₅IO₆ 10 LiOH 3.6 10.7 64.0 G Ex. 8 5 H₅IO₆ 10 NH₃ 0.4 7.4 37.0 E Ex. 9 10 H₅IO₆ 10 NH₃ 0.6 7.7 39.0 E Ex. 10 20 H₅IO₆ 10 NH₃ 1.2 7.7 49.9 E Ex. 11 30 H₅IO₆ 10 NH₃ 1.5 7.4 67.9 E C. Ex. 1 40 — 0 — 0 9.5 11.6 E C. Ex. 2 0 H₅IO₆ 10 — 0 1.2 0.5 E C. Ex. 3 40 — 0 NH₃ 4.5 10.4 14.0 E C. Ex. 4 40 — 0 triethanolamine 50 10.2 13.3 E C. Ex. 5 40 — 0 KOH 4.2 10.7 7.2 G C. Ex. 6 40 — 0 LiOH 1.8 10.8 8.8 G C. Ex. 7 40 — 0 H₂SO₄ 4.9 4.2 8.5 G C. Ex. 8 40 NaIO₄ 5 H₂SO₄ 7.4 1.9 13.3 G C. Ex. 9 40 H₅IO₆ 10 — 0 4.7 19.2 G C. Ex. 10 40 NaClO₂ 10 — 0 8.8 6.6 G C. Ex. 11 40 NaClO₃ 10 — 0 8.9 5.7 G C. Ex. 12 40 NaClO₄ 10 — 0 9.0 10.4 G C. Ex. 13 40 NaBrO₃ 10 — 0 9.1 10.9 G

TABLE 2 Object to be polished: Si [0001] plane of single crystal 4H—SiC substrate, 2 in. (50 mm) in diameter Polishing machine: Engis Japan, EJ-380IN Polishing composition supply rate: 50 mL/minute Polishing pressure: 500 g/cm² Press platen rotation speed: 80 rpm Head rotation speed: 30 rpm Polishing time: 4 hours Polishing pad: Nitta Haas Suba800

As shown in Table 1, each of the polishing compositions prepared in Examples 1 to 11 achieved a higher removal rate than that prepared in Comparative example 1, which corresponds to the composition disclosed by Japanese Laid-Open Patent Publication No. 2004-299018, and those prepared in Comparative examples 2 to 13. Moreover, each of the polishing compositions prepared in Examples 1 to 11 exhibited a satisfactory storage stability. The polishing compositions prepared in Comparative examples 10 to 13 were incorporated with sodium chlorite, sodium chlorate, sodium perchlorate and sodium bromate, respectively, in place of an iodine compound. Each of these compositions showed a lower removal rate than any of the polishing compositions prepared in Examples 1 to 11. These results indicate that a chlorine and bromine compounds, although belonging to halogen compounds like iodine compound, have no effect of enhancing polishing capability of polishing compositions for the Si [0001] plane.

Next, the third embodiment of the present invention will be described.

The polishing composition of the third embodiment is produced by incorporating colloidal silica sol with a given quantity of an iodine compound and, as required, diluting the resulting mixture with water. It is therefore essentially composed of colloidal silica working as abrasive grains, an iodine compound and water. The polishing composition of the third embodiment is used for polishing a single crystal 4H-SiC or 6H-SiC substrate, more specifically the C [000-1] plane of a single crystal 4H-SiC or 6H-SiC substrate.

The polishing composition of the third embodiment may fail to exhibit a sufficient polishing capability when it contains colloidal silica at below 0.05% by mass, or more specifically below 0.1% by mass, or even more specifically below 1% by mass. Therefore, it preferably contains colloidal silica at 0.05% by mass or more, more preferably 0.1% by mass or more, most preferably 1% by mass or more, in order to achieve a higher removal rate. On the other hand, it is uneconomical when it contains colloidal silica at above 40% by mass, or more specifically 35% by mass, or even more specifically more 30% by mass, because it shows a removal rate close to a saturation level, increasing only to a limited extent as silica content increases. Therefore, it contains colloidal silica preferably at 40% by mass or less, more preferably 35% by mass or less, most preferably 30% by mass or less.

Colloidal silica having an average primary particle size below 5 nm, or more specifically below 15 nm, or even more specifically below 25 nm does not have a sufficient polishing capability for the C [000-1] plane. Therefore, colloidal silica for the polishing composition of the third embodiment preferably has an average primary particle size of 5 nm or more, more preferably 15 nm or more, most preferably 25 nm or more, for the composition to achieve a higher removal rate. On the other hand, colloidal silica having an average primary particle size above 120 nm, or more specifically above 100 nm, or even more specifically above 85 nm has to be incorporated at a fairly high content in the polishing composition of the third embodiment, for the composition to exhibit a sufficient polishing capability. Therefore, it preferably has an average primary particle size of 120 nm or less, more preferably 100 nm or less, most preferably 85 nm or less, in order to reduce cost of the polishing composition. The average primary particle size of colloidal silica may be determined from the relative surface area measured by, e.g., the BET method.

The iodine compound incorporated in the polishing composition of the third embodiment to improve its removal rate may be an iodic acid, periodic acid or a salt thereof, preferably periodic acid or periodate, still more preferably orthoperiodic acid (H₅IO₆) or sodium metaperiodate (NaIO₄). A periodic acid and periodate are preferable to an iodic acid and iodate because of their higher redox potential and higher oxidizing power. Orthoperiodic acid and sodium metaperiodate are preferable to other periodic acids and periodates, because of their wider availability.

The polishing composition of the third embodiment may fail to polish the C [000-1] plane at a sufficiently high removal rate when it contains an iodine compound at below 0.1 g/L, or more specifically below 1 g/L, or even more specifically below 5 g/L. Therefore, it preferably contains an iodine compound at 0.1 g/L or more, more preferably 1 g/L or more, most preferably 5 g/L or more, in order to achieve a higher removal rate. On the other hand, when the polishing composition of the third embodiment contains an iodine compound at above 500 g/L, or more specifically above 250 g/L, or even more specifically above 100 g/L, it may deteriorate a polishing pad faster. Moreover, when the iodine compound contained in the polishing composition is an iodate or periodate, a precipitate may be formed. In order to avoid these problems, the polishing composition of the third embodiment contains an iodine compound preferably at 500 g/L or less, more preferably 250 g/L or less, most preferably 100 g/L or less.

It is essential that the polishing composition of the third embodiment is kept at a pH of 8 or less to suitably polish the C [000-1] plane. It should be noted, however, that the polishing composition tends to have a polishing capability decreasing as pH level increases, even when it is 8 or less. Therefore, the composition of the third embodiment is preferably kept at a pH of 7.5 or less, more preferably 7 or less in order to achieve a higher removal rate. On the other hand, when the polishing composition of the third embodiment is kept at an excessively low pH level, it may tend to corrode the polishing machine in which it is used. Therefore, it is preferably kept at a pH of 0.5 or more, more preferably 0.8 or more, most preferably 1 or more viewed from prevention of polishing machine corrosion.

The third embodiment brings the following advantages.

The polishing composition of the third embodiment can polish the C [000-1] plane of a single crystal 4H-SiC or 6H-SiC substrate faster than a conventional composition, conceivably by virtue of an iodine compound, which is iodic acid, periodic acid or a salt thereof, exhibiting a sufficient oxidizing power in a pH region of 8 or less to oxidize the plane.

Pits as one type of surface defects develop when portions easier to polish on an object to be polished are polished faster than portions more difficult to polish. Their development tends to be retarded as removal rate increases. Therefore, the polishing composition of the third embodiment, which can polish the C [000-1] plane faster than a conventional composition, leaves fewer pits on the polished C [000-1] plane.

The polishing composition of the third embodiment may be modified in the following manner.

Colloidal silica as the abrasive grains for the composition of the third embodiment may be replaced by another species of silica, e.g., fumed silica, or alumina or chromium oxide. However, silica (in particular colloidal silica) is preferably used, because it tends to leave less damage on the polished surface than alumina or chromium oxide.

Incorporation of colloidal silica is not essential for the polishing composition of the third embodiment. In other words, the polishing composition of the third embodiment may be essentially composed of an iodine compound and water. However, abrasive grains, e.g., those of colloidal silica, are preferably incorporated to achieve a higher removal rate.

The polishing composition of the third embodiment may be incorporated with one or more known additives, e.g., pH adjuster, corrosion inhibitor or defoaming agent.

The polishing composition of the third embodiment may be used for polishing a plane other than the Si [0001] or C [000-1] plane of a single crystal 4H-SiC or 6H-SiC substrate. Moreover, it may be used for polishing an object formed of single crystal cubic silicon carbide, e.g., a 3C-SiC substrate. It may be used to polish an object formed of single crystal silicon carbide other than a single crystal silicon carbide substrate, or an object other than the object formed of single crystal silicon carbide. However, it is stressed that the polishing composition of the third embodiment has a higher polishing capability for an object formed of single crystal silicon carbide, in particular the C [000-1] plane of a single crystal 4H-SiC or 6H-SiC substrate than a conventional composition. It is therefore preferably used for polishing an object formed of single crystal silicon carbide, in particular the C [000-1] plane of a single crystal 4H-SiC or 6H-SiC substrate.

Next, Examples and Comparative examples related to the polishing composition of the third embodiment will be described.

Abrasive grains, an iodine compound or alternative compound and pH adjuster were adequately mixed, and the mixture was diluted with water as required to prepare a polishing composition in each of Examples 101 to 116 and Comparative examples 101 to 111. These components are given in detail in Table 3 together with composition pH levels.

First, the C [000-1] plane of a single crystal silicon carbide substrate was polished preliminarily with a slurry containing abrasive grains of polycrystalline diamond (average particle size: 0.5 μm) under the polishing conditions given in Table 4. Then, the preliminarily polished plane was finished with the polishing composition prepared in each of Examples 101 to 116 and Comparative examples 101 to 111 under the polishing conditions given in Table 5. The substrate was weighed before and after the finishing polishing to determine removal rate, given in the “removal rate” column in Table 3.

Each of the finishing-polished C [000-1] planes was observed by an optical microscope (magnification: 50) to evaluate development of surface defects caused by the polishing composition in accordance with the following standards:

Good: No pit, step or roughened surface is observed, marked with G, and

Poor: Pit, step or roughened surface is observed, marked with P.

The results are given in Table 3 in the “surface conditions” column. No pit, step or roughened surface was observed by the optical microscope (magnification: 50) on the preliminarily polished C [000-1] plane.

TABLE 3 Iodine compound or alternative Abrasive grain compound pH adjuster Average primary Content Content Content Removal Surface Name particle size [nm] [mass %] Name [g/L] Name [g/L] pH rate [nm/hr.] conditions Ex. 101 — — 0 H₅IO₆ 10 — 0 1.8 100 G Ex. 102 colloidal silica 75 0.04 H₅IO₆ 10 — 0 1.1 196 G Ex. 103 colloidal silica 75 0.2 H₅IO₆ 10 — 0 1.3 242 G Ex. 104 colloidal silica 75 5 H₅IO₆ 10 — 0 2.3 300 G Ex. 105 colloidal silica 75 30 H₅IO₆ 10 — 0 2.5 307 G Ex. 106 colloidal silica 75 30 H₅IO₆ 5 — 0 2.8 190 G Ex. 107 colloidal silica 75 30 H₅IO₆ 20 — 0 2.0 352 G Ex. 108 colloidal silica 75 30 H₅IO₆ 50 — 0 1.3 295 G Ex. 109 colloidal silica 75 0.6 NaIO₄ 10 — 0 5.9 141 G Ex. 110 colloidal silica 75 5 NaIO₄ 10 — 0 6.7 171 G Ex. 111 colloidal silica 75 5 NaIO₄ 7 — 0 7.7 131 G Ex. 112 colloidal silica 75 30 NaIO₄ 10 H₂SO₄ 2 2.4 384 G Ex. 113 colloidal silica 16 1 H₅IO₆ 10 — 0 1.8 177 G Ex. 114 colloidal silica 38 1 H₅IO₆ 10 — 0 1.8 219 G Ex. 115 colloidal silica 75 1 H₅IO₆ 10 — 0 1.8 262 G Ex. 116 fumed silica 30 1 H₅IO₆ 10 — 0 1.8 183 G C. Ex. 101 colloidal silica 75 30 — 0 — 0 9.8 18 P C. Ex. 102 colloidal silica 75 30 H₂O₂ 5 — 0 9.5 7 P C. Ex. 103 colloidal silica 75 30 HNO₃ 6 — 0 1.8 21 P C. Ex. 104 colloidal silica 75 30 Al(NO₃)₃ 10 HNO₃ 6 1.6 19 P C. Ex. 105 colloidal silica 75 30 Fe(NO₃)₃ 10 HNO₃ 6 1.6 19 P C. Ex. 106 colloidal silica 75 30 0.5N HCl 0.5 — 0 1.8 13 P C. Ex. 107 colloidal silica 75 30 NaClO 10 — 0 9.3 26 P C. Ex. 108 colloidal silica 75 30 HclO₄ 10 — 0 1.0 14 P C. Ex. 109 colloidal silica 75 30 H₂O₂ 5 H₂SO₄ 2 2.0 11 P C. Ex. 110 colloidal silica 75 30 phosphonic 10 — 0 1.5 3 P acid C. Ex. 111 colloidal silica 75 30 malic acid 10 — 0 3.2 4 P

TABLE 4 Preliminary polishing Object to be polished: C [000-1] plane of single crystal 4H—SiC substrate, 2 in. (50 mm) in diameter Polishing machine: Nano Factor, FACT-200 Press platen rotation speed: 120 rpm Polishing time: 40 minutes Polishing pad: Nitta Haas Suba800

TABLE 5 Finishing polishing Object to be polished: Single crystal 4H—SiC substrate, 2 in. (50 mm) in diameter after preliminary polishing Polishing machine: Udagawa Optical Machine Co., Ltd., lens polishing machine (Oscar type) Polishing composition supply rate: 6.7 mL/minute Polishing pressure: 200 g/cm² Press platen rotation speed: 130 rpm Head rotation speed: 0 rpm Polishing time: 2 hours Polishing pad: Nitta Haas Suba800

As shown in Table 3, each of the polishing compositions prepared in Examples 101 to 116 achieved a higher removal rate than that prepared in Comparative example 101, which corresponds to the composition disclosed by Japanese Laid-Open Patent Publication No. 2004-299018, and those prepared in Comparative examples 102 to 111. Moreover, each of the polishing compositions prepared in Examples 101 to 116 gave satisfactory conditions of the polished surface.

The fourth embodiment of the present invention will be described.

The polishing composition of the fourth embodiment is produced by incorporating colloidal silica sol with a given quantity of an iodine compound and, as required, diluting the resulting mixture with water. It is therefore essentially composed of colloidal silica working as abrasive grains, an iodine compound and water. The polishing composition of the fourth embodiment is used for polishing a single crystal 4H-SiC or 6H-SiC substrate, more specifically a single crystal silicon carbide substrate composed of the Si [0001] plane and C [000-1] plane on each side. When it polishes these planes sequentially, the order does not matter.

The polishing composition of the fourth embodiment may fail to exhibit a sufficient polishing capability when it contains colloidal silica at below 0.05% by mass, or more specifically below 0.1% by mass, or even more specifically below 1% by mass. Therefore, it preferably contains colloidal silica at 0.05% by mass or more, more preferably 0.1% by mass or more, most preferably 1% by mass or more, in order to achieve a higher removal rate. On the other hand, it is uneconomical when it contains colloidal silica at above 40% by mass, or more specifically above 35% by mass, or even more specifically above 30% by mass, because it shows removal rate close to a saturation level, increasing only to a limited extent as silica content increases. Therefore, it contains colloidal silica preferably at 40% by mass or less, more preferably 35% by mass or less, most preferably 30% by mass or less.

Colloidal silica having an average primary particle size below 5 nm, or more specifically below 15 nm, or even more specifically below 25 nm does not have a sufficient polishing capability for the Si [0001] and C [000-1] planes. Therefore, colloidal silica for the polishing composition of the fourth embodiment preferably has an average primary particle size of 5 nm or more, more preferably 15 nm or more, most preferably 25 nm or more, for the composition to achieve a higher removal rate. On the other hand, colloidal silica having an average primary particle size above 120 nm, or more specifically above 100 nm, or even more specifically above 85 nm has to be incorporated at a fairly high content in the polishing composition of the fourth embodiment, for the composition to exhibit a sufficient polishing capability. Therefore, it preferably has an average primary particle size of 120 nm or less, more preferably 100 nm or less, most preferably 85 nm or less, in order to reduce cost of the polishing composition. The average primary particle size of colloidal silica may be determined from the relative surface area measured by, e.g., the BET method.

The iodine compound incorporated in the polishing composition of the fourth embodiment to improve its removal rate may be an iodic acid, periodic acid or a salt thereof, preferably periodic acid or periodate, more preferably orthoperiodic acid (H₅IO₆) or sodium metaperiodate (NaIO₄). A periodic acid and periodate are preferable to an iodic acid and iodate because of their higher redox potential and higher oxidizing power. Orthoperiodic acid and sodium metaperiodate are preferable to other periodic acids and periodates, because of their wider availability.

The polishing composition of the fourth embodiment may fail to polish the Si [0001] and C [000-1] planes at a sufficiently high removal rate when it contains an iodine compound at below 0.1 g/L, or more specifically below 1 g/L, or even more specifically below 5 g/L. Therefore, it preferably contains an iodine compound at 0.1 g/L or more, more preferably 1 g/L or more, most preferably 5 g/L or more, in order to achieve a higher removal rate. On the other hand, when the polishing composition of the fourth embodiment contains an iodine compound at above 500 g/L, or more specifically above 250 g/L, or even more specifically above 100 g/L, it may deteriorate a polishing pad faster. Moreover, when the iodine compound contained in the polishing composition is an iodate or periodate, a precipitate may be formed. In order to avoid these problems, the polishing composition of the fourth embodiment contains an iodine compound preferably at 500 g/L or less, more preferably 250 g/L or less, most preferably 100 g/L or less.

The polishing composition of the fourth embodiment may have an insufficient polishing capability for the Si [0001] plane when kept at a pH level below 6, and an insufficient polishing capability for the C [000-1] plane when kept at a pH level above 8. Therefore, it is essential that the polishing composition of the fourth embodiment is kept at a pH of 6 to 8, inclusive to suitably polish both planes.

The fourth embodiment brings the following advantage.

the polishing composition of the fourth embodiment can polish each of the si [0001] and C [000-1] planes of a single crystal 4H-SiC or 6H-SiC substrate at a high removal rate, conceivably by virtue of an iodine compound, which is iodic acid, periodic acid or a salt thereof, exhibiting a sufficient oxidizing power in a pH region of 6 to 8, inclusive to oxidize these planes.

The polishing composition of the fourth embodiment may be modified in the following manner.

Colloidal silica as the abrasive grains for the composition of the fourth embodiment may be replaced by another species of silica, e.g., fumed silica, or alumina or chromium oxide. However, silica (in particular colloidal silica) is preferably used, because it tends to leave less damage on the polished surface than alumina or chromium oxide.

The polishing composition of the fourth embodiment may be incorporated with one or more known additives, e.g., pH adjuster, corrosion inhibitor or defoaming agent.

The polishing composition of the fourth embodiment may be used for polishing an object having the Si [0001] and C [000-1] planes mixed on one side, instead of one plane for each side. Moreover, it is applicable to a plane other than the Si [0001] or C [000-1] plane of a single crystal 4H-SiC or 6H-SiC substrate, to an object formed of single crystal cubic silicon carbide, e.g., a 3C-SiC substrate, or, still more, to an object formed of single crystal silicon carbide other than a single crystal silicon carbide substrate or an object other than the object formed of single crystal silicon carbide. However, given that the polishing composition of the fourth embodiment has a capability of polishing each of the Si [0001] and C [000-1] planes at a high removal rate, it is preferably used for polishing these planes simultaneously or one by one, or these planes mixed on one side.

Next, Examples and Comparative examples related to the polishing composition of the fourth embodiment will be described.

Colloidal silica sol, orthoperiodic acid as an iodine compound and ammonia as a pH adjuster were adequately mixed, and the mixture was diluted with water as required to prepare a polishing composition in each of Examples 201 to 205 and Comparative examples 201 to 205. These components are given in detail in Table 6 together with composition pH levels. The colloidal silica for these compositions had an average primary particle size of 38 nm.

The Si [0001] and C [000-1] planes of a single crystal silicon carbide substrate were polished with the polishing composition prepared in each of Examples 201 to 205 and Comparative examples 201 to 205 under the polishing conditions given in Table 7. The substrate was weighed before and after the polishing of Si [0001] plane to determine removal rate for Si [0001] plane, given in the “removal rate for Si [0001] plane” column in Table 6. The substrate was weighed before and after the polishing of C [000-1] plane to determine removal rate for C [000-1] plane, given in the “removal rate for C [000-1] plane” column in Table 6.

The Si [0001] and C [000-1] planes of a single crystal silicon carbide substrate, polished with each of Examples 201 to 205 and Comparative examples 201 to 205 under the polishing conditions given in Table 7, were observed by an optical microscope (magnification: 50) to evaluate development of surface defects caused by the polishing composition in accordance with the following standards:

Good: no pit, step or roughened surface is observed, marked with G, and

Poor: pit, step or roughened surface is observed, marked with P.

The results are given in Table 6 in the “surface conditions of Si [0001] plane” column and “surface conditions of C [000-1] plane” column.

TABLE 6 Colloidal Removal Surface Removal Surface silica Orthoperiodic Ammonia rate for Si conditions rate for C conditions content acid content content [0001] plane of Si [0001] [000-1] plane of C [000-1] [mass %] [g/L] [g/L] pH [nm/hr.] plane [nm/hr.] plane C. Ex. 201 10 10 0 2.2 7.4 G 506 G C. Ex. 202 10 10 0.6 4.3 7.5 G 391 G Ex. 201 10 10 0.7 6.0 10.5 G 343 G Ex. 202 10 10 0.8 6.6 14.3 G 326 G Ex. 203 10 10 0.9 7.0 17.5 G 279 G Ex. 204 20 10 0.9 7.0 19.1 G 281 G Ex. 205 10 10 1.4 7.6 20.1 G 265 G C. Ex. 203 10 10 1.8 8.2 22.5 G 248 P C. Ex. 204 10 10 7.5 9.9 30.9 G 154 P C. Ex. 205 10 0 0 9.5 7.2 G 36 P

TABLE 7 Object to be polished: Si [0001] and C [000-1] planes of single crystal 4H—SiC substrate, 2 in. (50 mm) in diameter Polishing machine: Engis Japan, EJ-380IN Polishing composition supply rate: 50 mL/minute Polishing pressure: 230 g/cm² Press platen rotation speed: 80 rpm Head rotation speed: 30 rpm Polishing time: 4 hours Polishing pad: Nitta Haas Suba800

As shown in Table 6, each of the polishing compositions prepared in Examples 201 to 205 gave good surface conditions on each of the Si [0001] and C [000-1] planes, and achieved high removal rates for both planes. 

1. A method for polishing an object formed of single crystal silicon carbide, comprising: preparing a polishing composition containing abrasive grains; an iodine compound selected from the group consisting of an iodic acid, a periodic acid, and a salt thereof; and water, and having a pH of 6 or more; and polishing the object using the polishing composition.
 2. The method according to claim 1, wherein the object to be polished is a substrate having a side composed of a Si[0001] plane, and wherein the polishing of the object includes polishing the side of the substrate using the polishing composition.
 3. The method according to claim 1, further comprising, prior to the polishing of the object using the polishing composition, preliminarily polishing the object using a slurry containing abrasive grains of diamond.
 4. The method according to claim 1, wherein the abrasive grains are colloidal silica.
 5. The method according to claim 4, wherein the colloidal silica has an average primary particle size of from 25 to 85 nm.
 6. The method according to claim 1, wherein the iodine compound is orthoperiodic acid, H₅IO₆, or sodium metaperiodate, NaIO₄.
 7. The method according to claim 1, wherein the polishing composition further contains lithium hydroxide, an inorganic lithium salt, or ammonia.
 8. The method according to claim 1, wherein the polishing composition has a pH of from 7 to
 12. 9. A method for polishing an object formed of single crystal silicon carbide, comprising: preparing a polishing composition containing an iodine compound selected from the group consisting of an iodic acid, a periodic acid, and a salt thereof; and water, and having a pH of 8 or less; and polishing the object using the polishing composition.
 10. The method according to claim 9, wherein the object to be polished is a substrate having a side composed of a C[000-1] plane, and wherein the polishing of the object includes polishing the side of the substrate using the polishing composition.
 11. The method according to claim 9, further comprising, prior to the polishing of the object using the polishing composition, preliminarily polishing the object using a slurry containing abrasive grains of diamond.
 12. The method according to claim 9, wherein the polishing composition further contains abrasive grains.
 13. The method according to claim 12, wherein the abrasive grains are colloidal silica.
 14. The method according to claim 13, wherein the colloidal silica has an average primary particle size of from 25 to 85 nm.
 15. The method according to claim 9, wherein the iodine compound is orthoperiodic acid, H₅IO₆, or sodium metaperiodate, NaIO₄.
 16. The method according to claim 9, wherein the polishing composition has a pH of from 1 to
 7. 17. A method for polishing an object formed of single crystal silicon carbide, comprising: preparing a polishing composition containing abrasive grains; an iodine compound selected from the group consisting of an iodic acid, a periodic acid, and a salt thereof; and water, and having a pH of 6 to 8; and polishing the object using the polishing composition.
 18. The method according to claim 17, wherein the object to be polished is a substrate having a first side composed of a Si[0001] plane and a second side composed of a C[000-1] plane, and wherein the polishing of the object includes polishing, using the polishing composition, the first surface and the second surface of the substrate simultaneously or one by one.
 19. The method according to claim 17, further comprising, prior to the polishing of the object using the polishing composition, preliminarily polishing the object using a slurry containing abrasive grains of diamond.
 20. The method according to claim 17, wherein the abrasive grains are colloidal silica.
 21. The method according to claim 20, wherein the colloidal silica has an average primary particle size of from 25 to 85 nm.
 22. The method according to claim 17, wherein the iodine compound is orthoperiodic acid, H₅IO₆, or sodium metaperiodate, NaIO₄. 